Filler reinforced thermoset polymers are used in molding applications requiring high strength, lightweight, dimensional stability, and corrosion resistance. They are generally molded in matched die sets where heat and pressure are applied to simultaneously cure or crosslink the polymer constituents and shape the desired article. Such articles have frequently replaced assemblies of several stamped or cast metal parts.
Almost all of the thermoset polymer compositions now used in making reinforced plastic components contain crosslinkable polyester resins. However, other viscous resin systems based on polymers such as epoxies, phenolics, silicones, phthalates, polyurethanes, etc., are used where their special properties suit a particular application and may be employed in the practice of this invention.
Sheet molding compound (SMC) is generally defined as a composite molding material made up of an unsaturated polyester resin, a low profile thermoplastic polymer agent, an inert filler, a fibrous reinforcement, metal oxide maturation aids and processing aids. Generally, the unsaturated polyester resin is the reaction product of polypropylene oxide, phthalic anhydride and maleic anhydride. These constituents are typically provided in solution with styrene monomer. Low profile agents that include such thermoplastic polymers as polymethylmethacrylate, polyvinyl acetate or polybutadienestyrene copolymer rubber are added to improve surface smoothness. The inert filler is usually calcium carbonate, i.e., ground limestone. The fibrous filler is generally chopped fiberglass in roving lengths of about 1/2 to 2 inches, each roving containing several hundred individual fibers loosely bound together by a starchlike binder
SMC is generally made at a machine specifically designed for the purpose. A sheet of SMC is produced by applying a thin layer of a low viscosity mixture of resin-styrene monomer and the other ingredients onto a continuous, styrene-impermeable, carrier film of polyethylene or other suitable material. The glass fibers are then laid on top of the resin layer. A second layer of the resin mixture is applied over the fiberglass layer and a second protective film is laid on top of it. The resulting sandwich of resin and glass between protective carrier films is then carefully rolled to knead the glass fiber into the resin. This mixing action must be very gentle as the viscosity of the constituents is low at this time. Application of any substantial compression would cause the resin mixture to ooze out between the edges of the protective films.
The final step of SMC preparation is to coil the just-made compound into rolls, and allow them to maturate under controlled temperature conditions (preferably near room temperature, .about.23.degree. C.) for several days. SMC maturation is generally defined as the process in which the metal oxide constituents react with free organic acid groupings of the polyester constituent to thicken it. Maturation is allowed to proceed to a point where the viscosity of the material has increased to the extent that it can be readily handled for molding and the surface is not too tacky. While metal oxides and hydroxides are the predominant maturation agents, isocyanates are sometimes used as thickeners via a urethane reaction with the polyester component.
To compression mold maturated SMC, the protective films are first stripped from the compound. The resin-fiber composite, generally 2 to 8 mm in thickness, is then cut into suitably sized pieces and stacked into a "charge" of the proper geometry for a specific mold. The charge is placed into a matched metal die mold in a suitable compression molding press. Under typical molding conditions, a mold temperature of about 150.degree. C. is maintained, a molding pressure of about 3.4 to 10.5 megaPascals is applied, and the article is cured in place for about 1 to 3 minutes.
Problems have been encountered in making reliable SMC parts for rigorous structural applications. For example, tensile strength measurements made on a large molded SMC panel may vary widely and unpredictably by location. Furthermore, the tensile strength of molded SMC parts tends to be anisotropic, the measured strength being higher in the direction of material flow in the mold. As a consequence, molded SMC parts may not be useful in applications where consistent and isotropic strength characteristics are required.
Another serious problem with compression molded SMC is a prevalence of microscopic surface defects in the form of small holes or pores at or near the surface of a molded part. These cannot be tolerated where a high quality painted surface is required. When paint is applied to a part and cured at an elevated temperature, outgassing from the pores creates pops and craters in the painted surface. Another prevalent problem where high gloss paints are applied is low distinctness of image. Distinctness of image (DOI) is a measurement of the resolution of images reflected from a glossy painted surface. Low DOI is generally caused by microscopic irregularity of the substrate or the paint itself. SMC, as molded, generally exhibits a less than desirable DOI. Another common problem with SMC parts is surface waviness which detracts significantly from the appearance of high gloss coatings.
Many attempts have been made to improve the physical properties and the surface characteristics of SMC by modifying the polymer chemistry, filler loadings and fiber reinforcements. Better surfacing can be achieved by incorporating a high proportion of low profile thermoplastic resin. However, this is relatively expensive and adversely affects the strength of the compositions. Relatively expensive pliable fiber fillers such as graphite can be substituted for chopped fiberglass but these provide only minimal improvements in tensile characteristics and surface appearance. The overall chemistry of SMC compositions can be changed, but each such change dictates extensive study and testing to assure that the new material is adequate for a particular application. Moreover, in most cases changing the chemistry or filler content of an SMC composition provides an improvement of only a few percent in either the surface characteristics or tensile properties.